fix(review): CSP nonce for FOUC script + feed_delta truncation signal

Two items surfaced by the branch re-review:

CSP regression (blocking): script-src 'self' blocked the template-authored
anti-FOUC theme script in app.html — SvelteKit's mode:'auto' only hashes
scripts it injects. Added nonce="%sveltekit.nonce%" so it's substituted per
request and included in the CSP (survives future edits, unlike a pinned hash).
Verified: emitted CSP nonce matches the script tag; no violation, no flash.

feed_delta silent truncation: the LIMIT 200 (added earlier to kill the
stale-`since` DoS) returned only the newest slice with no signal, so a client
that missed 200+ uploads during a reconnect could not tell it should full-
refresh — the older missed uploads were dropped and unrecoverable (the next
delta advances `since` past them). DeltaResponse now carries `truncated`; the
feed's feed-delta handler calls loadFeed(true) to resync from page 1 instead
of merging a partial slice.

Verified: cargo check clean, svelte-check 0 errors, production build green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/routes/feed/+page.svelte

@@ -206,6 +206,13 @@
 			onSseEvent('feed-delta', (data) => {
 				try {
 					const delta = JSON.parse(data) as DeltaResponse;
+					if (delta.truncated) {
+						// Missed more than the backend delta cap while backgrounded — the
+						// delta is only the newest slice, so merging would leave a silent gap
+						// of older-but-still-new uploads. Resync from page 1 instead.
+						void loadFeed(true);
+						return;
+					}
 					if (delta.uploads.length) {
 						const seen = new Set(uploads.map((u) => u.id));
 						const fresh = delta.uploads.filter((u) => !seen.has(u.id));